hair particles disappear when joining objects
Joining the meshes is making the Particle modifier disappear.
When you join two meshes the active object modifiers are preserved. The active object is the last one you selected, and will be outlined in a lighter orange. In your case you're selecting the eye last, and its modifiers are being preserved, not the eyebrow's ones.
You need to select the eyebrow mesh last, then join.
Also, to avoid hair getting scattered everywhere when you join the meshes do the following before joining:

Select the eyebrow, go into Edit mode.
Select the area that should have hair.
In the Object Data panel, create a new Vertex Group, rename it to your liking and click on &quot;Assign&quot;.
In the Particle panel, scroll down to the Vertex Groups and set the vertex group you created as Density.
Now you can leave edit mode and join the meshes.